Rolling bearings are widely used in various mechanical applications, including automotive and industrial applications. Grounding devices are used in a variety of bearing applications, including AC or DC electrical motors, generators, and other applications having rotating shafts. In these applications, the grounding devices are used to prevent shaft induced currents from causing electrical discharge machining (EDM) due to electrical currents passing through a rolling bearing component. EDM causes pitting, fluting, and fusion craters on the rolling bearing components resulting in premature rolling bearing failure. Known EDM prevention methods include di-electric ceramic bearing coatings, Faraday shields, electrically conductive bearing grease, and shaft-contacting ground brushes.
Di-electric ceramic bearing coatings include a hard brittle coating that can fracture during installation or in use. Cracks in the coating allows current to pass through the bearing which results in EDM. Di-electric ceramic bearing coatings also have limited di-electric strength and at high voltage potential, current can pass through the coating structure compromising the EDM protection of the coating. Faraday shields are also known, as disclosed in U.S. Pat. No. 7,878,814. Faraday shields are expensive to implement in most rolling bearing applications. Electrically conductive grease or liquid metal, such as disclosed in U.S. Pat. No. 8,248,725, are used to allow current to continuously pass through bearing surfaces to prevent arcing that causes EDM, but may contain elements such as copper, carbon, or phosphorous which can cause excessive wear on the bearing surfaces and lead to premature failure of the rolling bearing.
Spring loaded shaft grounding brushes are also known and allow a continuous flow of current to ground. Known grounding brushes are prone to excessive wear, material transfer, and oxide formation on the mating surface. These oxides may become a di-electric and may prevent effective current transfer from the shaft to the ground through the brushes. This can cause electrical arcing and lead to EDM damage of the shaft. Once the impedance of the shaft grounding brush exceeds the impedance of the bearings, the current can also seek ground through the bearings which can cause EDM damage to the bearings. Shaft grounding brushes may also vibrate due to imperfections in the shaft surface, i.e. an out of round condition. When the grounding brush momentarily loses contact with the shaft during vibration, the current can arc from the grounding brush to the shaft causing EDM damage. Alternatively, the current can flow through the bearings if the voltage potential is sufficiently high while the brush loses contact with the shaft during vibration. Traditional spring loaded shaft grounding brushes are prone to rapid and excessive wear and can ultimately lose contact with the shaft causing the current to seek ground through the bearing and causing EDM damage in the bearing. Known bearing assemblies with EDM protection include separately formed seal assemblies as part of the bearing to retain lubricant and prevent the ingress of debris, and separately mounted grounding brush assemblies, which require significant axial space and increases the overall space requirements of the rolling bearing assembly.
As explained above, known EDM protection for bearings are either too expensive, unreliable, too large, or can cause damage to the bearing components.